1. Field of the Invention
This invention relates to a method and apparatus for power generation. In one aspect, this invention relates to the use of gas turbines for electric power generation. In one aspect, this invention relates to the use of partial oxidation gas turbines for power generation. In one aspect, this invention relates to a method and apparatus for power generation employing fuel and air staging.
2. Description of Related Art
Gas turbines are one of the major sources for power generation in use today. However, the best efficiency for gas turbine power generation achieved to date by single cycle conventional gas turbines is only about 38%. One significant drawback of using gas turbines is that a significant portion, approximately 62-75%, of the fuel energy input to the gas turbines is lost in the turbine exhaust. This exhaust energy is in the form of thermal energy only, which makes it difficult to use for effective power generation. Staged reheat gas turbines have the capability to improve efficiency and, in some gas turbines, fuel staging has been employed, resulting in improvements to system efficiency.
A combined cycle is characteristic of a power producing engine or plant that employs more than one thermodynamic cycle. In a combined cycle power plant or combined cycle gas turbine plant, a gas turbine generator generates electric power and the waste heat is used in a bottoming cycle to make steam to generate additional electric power by means of a steam turbine, thereby enhancing the overall efficiency of the electric power generation. At present, net plant efficiency for conventional gas turbine combined cycles is about 58-59% of the lower heating value (LHV), and additional increases in efficiency are limited by several factors. Firstly, gas turbines are operated conventionally with large amounts of excess air, and significant amounts of thermal energy are transferred from the gas turbine exhaust to the bottoming cycle. Secondly, gas turbine exhaust temperatures usually do not exceed 1100° F., which restricts the superheated steam temperature obtainable in the bottoming cycle and, thus, limits the efficiency of the bottoming cycle to about 35-37%. Thirdly, air and/or fuel recuperation is not applied conventionally in combined cycles because the effect of recuperation is to reduce the temperature of the flue gases entering the bottoming cycle, thereby significantly decreasing the bottoming cycle efficiency as well as the overall system efficiency.
U.S. Pat. No. 2,675,672 teaches a gas turbine plant employing fuel and air staging. Fuel staging is performed in series where 100% of the fuel is supplied to a gas producer from which a portion of the gaseous fuel is supplied to a first topping stage and another portion of the fuel is supplied to a second stage combustion process. Both stages are complete combustion stages; however, power is generated only from the second stage. U.S. Pat. No. 6,609,582 teaches a power generation system and method employing a hybrid electric powertrain having an engine configured to produce reformate to feed a solid oxide fuel cell. The system employs air staging in series, but no fuel staging. See also U.S. Pat. No. 7,421,835 which teaches an air-staged reheat power generation system. None of these systems can provide a combined cycle having efficiencies greater than 60% because there are no provisions for high temperature steam superheating and, therefore, high efficiency in the bottoming cycle cannot be achieved.